Process for producing glass coatings for anodic bonding purposes

ABSTRACT

The invention relates to a method of anodic bonding and a method for producing glass coatings for purposes of anodic bonding and colloidal solutions suitable for this purpose. The method is particularly suitable for anodic bonding with glass coatings with a thickness of greater than 100 nm up to 10 μm, which are used for connecting two semiconductor layers. In order to produce the required glass coating, an SiO 2  -sol is dissolved in at least one or in a mixture of n-alkanols (n=1 to 5). Then tetraethyl orthosilicate (TEOS), methyl triethoxysilane (MTEOS) and water are added to this organosol. Furthermore, a small quantity of acid is added to the desired quantity of alkali salts, and the colloidal solution is at least partly polymerized. The colloidal solution thus obtained is suitable for coating conductive materials by cost-effective methods such as immersion, spin-on deposition or spraying and subsequent tempering. After one coating there results a glass coating with a thickness of up to 2 μm. When the method is repeated a plurality of times, coating thicknesses of over 10 μm can be obtained.

This is a national stage application of PCT/DE96/00103, filed Jan. 17,1996.

The invention relates to a method of producing connections between analkali-containing glass and an electrically conductive material, forexample a metal or a semiconductor. Such connections are in particularsuitable for connecting the surfaces of two semiconductor elements, forexample two silicon wafers with an intermediate alkali-containing glasscoating. These methods are used above all in the semiconductor industry.

By anodic bonding is understood a method of producing a connectionbetween alkali-containing glass and electrically conductive material.Anodic bonding is particularly suitable for connecting a semiconductorplatelet with a glass layer. In this case, at a temperature of 300° C.to 450° C. an electrical voltage is applied between the ion-conductivealkali-containing glass and the semiconductor, for example silicon. Thiselectrical voltage causes electrochemical reactions at the boundarylayer between glass and semiconductor, by means of which the glass issecurely connected to the semiconductor. A decisive factor for anodicbonding is substantially the alkali content of the glass coating. Theresult is a secure undetachable connection between the alkali-containingglass and the semiconductor material.

For anodic bonding of silicon wafers, particularly suitable are boronsilicate glasses with a variable content of B₂ O₃, SiO₂ and/or Al₂ O₃.They are characterised by high chemical strength and by a lowcoefficient of expansion.

The glass coatings required for anodic bonding are conventionallyproduced by molecular coating processes such for example as sputteringor vapour deposition or by coating with appropriate colloidal solutions.In the molecular coating processes however, at larger coatingthicknesses such as for example required for connecting to siliconwafers, process times of some hours result. In addition the productionof such coatings in sputtering or vapour deposition installations alwaysinvolves the formation of such thick coatings inside the apparatus.These coatings must be removed from time to time, leading to frequentservicing and cleaning operations. Therefore the application of glasscoatings by molecular coating processes such as sputtering or vapourdeposition are expensive and unprofitable for anodic bonding with glassintermediate layers.

In contrast, glass coatings produced by spin-on deposition of orimmersion in appropriate colloidal solutions, are characterised in thatthe coating process and the apparatus required therefor provide goodvalue. For this purpose sol-gel-coatings on the basis of TEOS(tetraethyl orthosilicate) are normally used. When this coating methodis used, however, at the required coating thicknesses for anodic bondingof two semiconductor panels, mechanical stresses occur within the layerswhich lead to the formation of cracks. Therefore the normally usedsol-gel-coatings on the basis of TEOS cannot be used for the productionof coating thicknesses which exceed 100 nm.

In summary it can be seen that in order to produce glass coatings foranodic bonding with a thickness between 0.1 and 10 μm, as are requiredfor example for anodic bonding of two semiconductor plates, molecularapplication methods such as sputtering or vapour deposition areextremely expensive and involve a high outlay on servicing and cleaning,while the more cost-effective methods in which appropriate colloidalsolutions are applied for example by spin-on deposition or immersion,are not suitable for the production of such thick glass coatings. Thusuntil now there has been no suitable and at the same time cost-effectivemethod of producing alkali-containing glass coatings with a thicknessbetween about 0.1 and about 10 μm.

It is therefore the object of the invention to make available a simpleand cost-effective method for anodic bonding, in which alkali-containingglass coatings with a thickness exceeding 100 nm and can also extend asfar as about 10 μm, are used on an electrically conductive carriermaterial such for example as semiconductor wafers of silicon. It is afurther object of the invention to propose appropriate colloidalsolutions which can be used to produce such glass coatings by the methodaccording to the invention.

For the method according to the invention for anodic bonding, the glasscoating according to the invention is produced on the electricallyconductive material by firstly producing an SiO₂ -sol in at least one oralso in a mixture of a plurality of n-alkanols, n having a value between1 and 5. There is added to this organosol tetraethyl orthosilicateand/or methyl triethoxysilane. A small quantity of acid and then waterand the desired quantity of alkali salts are added, both the acid andthe alkali salts being capable of dissolution in an n-alkanol with n=1to 5. Thereupon this solution is polymerised over a lengthy period. Analmost clear opalescent solution is obtained which is extremely suitablefor coating. With this solution, in particular high proportions ofalkali salts up to the percentage range can be added without thesolution precipitating. This colloidal solution according to theinvention has a long storage life and can be stored for many hours andthen further processed without loss of quality. It is applied to theelectrically conductive material to be coated in an otherwise known way,for example by immersion, spin-on deposition or spraying. The coating isdried and cured (tempered) at a temperature which can be selected inaccordance with the desired properties of the glass coating. Once thismethod has been carried out, coating thicknesses of 1 to 2 μm can beachieved. In order to achieve larger coating thicknesses such forexample as 2 to 10 μm, this method can be repeated a number of times,the newly applied layer in each case having to be dried and temperedbetween the individual coating steps.

Following the last tempering of the glass coating, the electricallyconductive carrier material can be connected (bonded) with the glasscoating thus produced by means of application of an electrical current,which flows via the boundary layer between the electrically conductivematerial and the glass coating.

The method according to the invention is suitable for anodic bonding ofthe glass coating produced in accordance with the method according tothe invention to any electrically conductive material, particularlysemiconductor materials such for example as silicon wafers.

As a result of the method according to the invention there is nowavailable a simple and cost-effective method of providing electricallyconductive carrier materials with alkali-containing glass coatings,which can reach a thickness of up to 10 μm, and connecting themtherewith. It is particular advantage that, in order to produce theglass coating from a sol, the relatively simple and cost-effectiveapplication methods such as spin-on deposition, spraying or immersioncan be used for this purpose. The alkali-containing glass coatingproduced is to a large extent free of cracks and is extremely suitablefor anodic bonding. In particular, two coatings of electricallyconductive material, such for example as two silicon wafers, can besecurely and undetachably connected together by a glass coating producedaccording to the invention.

A further advantage of the method according to the invention is that theconcentration of the alkali ions in the finished glass coating can beadjusted within wide ranges by the addition of differing quantities ofalkali salts to the colloidal solution according to the invention.

Advantageous further developments of the method according to theinvention and of the colloidal solutions according to the invention areillustrated in the secondary claims.

It has proved advantageous to use as alkali salts sodium compounds,which can be advantageously dissolved in an n-alkanol with n=1 to 5, forexample in ethanol and/or methanol. Acetic acid has proved particularlysuitable as an acid for addition. This also is advantageously dissolvedin an n-alkanol with n=1 to 5, for example in ethanol, before additionto the colloidal solution.

The concentration of the colloidal solution allows the thickness of thefinished glass coating to be regulated. Where a thinner glass coating isto be produced in one coating step, the colloidal solution can bediluted before coating with n-alkanols, n lying between 1 and 5. In asimilar way, the colloidal solution can be concentrated, so that thecoating thickness of the individual coating is increased. The n-alkanolwhich is the solvent of the colloidal solution is advantageously usedfor dilution.

The properties of the finished glass coating are greatly influenced bythe temperature at which the applied and dried colloidal solution istempered. If tempering is carried out at a temperature beneath 400° C.,a large proportion of organic material, for example in the form ofmethyl groups, remains in the finished glass. Consequently, the finishedglass involves organically modified silicates with additional alkaliions.

If the glass coating is tempered at temperatures above 450° C. forexample in air or oxygen, the glass coating is practically totallycondensed.

If tempering is carried out at temperatures above 650° C., an extremelydense glass coating results, which has an Na₂ O-content, which can beextremely precisely predetermined by the addition of correspondingquantities of alkali salts to the colloidal solution. Such dense glasscoatings are characterised by extremely high tensile strength, and arethemselves resistant to alkaline etching.

Irrespective of the temperature at which the glass coating has beentempered, and irrespective of the number and/or sequence of temperingphases, all the glass coatings produced by the method according to theinvention bond extremely well.

Improvements to or alterations in the properties of the glass coatingproduced according to the invention can be achieved by the addition offurther chemical compounds to the colloidal solution according to theinvention. For example, in order to improve the chemical strength and inorder to adapt the coefficient of expansion of the glass coating to thematerials to be bonded, boron compounds such as boric acid and/ororganic aluminium compounds can be added, without impairing thefunctional properties of the finished glass coating important for anodicbonding.

Some particularly preferred and particularly simple embodiments of theinvention given by way of example will be shown in the following,without restricting the scope of the invention to these embodimentsgiven by way of example.

EXAMPLE 1

Starting with an aqueous solution of silicon dioxide particles with anaverage diameter of 7 nm, an ethanolic alcohol sol was produced. 35.6 gof methyl triethoxysilane and 11.5 g tetraethyl orthosilicatetriethoxysilane and 11.5 g of tetraethyl orthosilicate were added to 50g of this alcohol sol, the pH value of which was 2. Then 10 g water and16 g 10% sodium acetate solution in methanol were added with constantstirring. This mixture was then tempered for 36 to 38 hours at 22° C.

After termination of the reaction an almost clear but opalescentsolution is obtained, which is suitable for coating by immersion,spin-on deposition or spraying. After coating a silicon carrier materialwith this solution, the coating was dried and finally tempered at atemperature beneath 400° C.

A glass coating was obtained which contained a relatively highproportion of organic material (methyl groups). This material producedwas extremely suitable for anodic bonding and, after bonding effected inthe previous way by an electrical current, resulted in a secure bondbetween the silicon carrier material and the glass coating.

EXAMPLE 2

A glass coating was produced as in Example 1 and then tempered attemperatures above 450° C. This led to a further condensation of theglass coating.

EXAMPLE 3

A glass coating was produced as in Example 2 and then tempered at above650° C. An extremely dense glass coating resulted. This coating hadparticularly high tensile strength and proved resistant against alkalineetching by a 25% tetramethyl ammonium hydroxide solution. Suchtetramethyl ammonium hydroxide solutions are in particular used in themanufacture of micromechanical components. The glass coating produced inthis example is therefore particularly suitable for use in industrialproduction of such components.

We claim:
 1. A method for anodic bonding of an alkali-containing glassto an electrically conductive material comprising(a) providing acolloidal solution obtained by: producing a SiO₂ -sol in one or in amixture of n-alkanols, n having a value between 1 and 5;adding at leastone chemical compound selected from the group consisting of TEOS(tetraethyl orthosilicate) and MTEOS (methyl triethoxysilane) and anacid to the colloidal solution; adding water and alkali salts to thecolloidal solution; and at least partly polymerising the colloidalsolution; (b) applying the colloidal solution onto an electricallyconductive material to form an alkali-containing glass coating on saidelectrically conductive material; (c) drying and tempering saidalkali-containing glass coating; and (d) applying an electrical currentacross a boundary between the electrically conductive material and thealkali-containing glass coating to anodically bond the electricallyconductive material to the alkali-containing glass coating.
 2. Themethod according to claim 1, wherein the alkali salts are sodiumcompounds.
 3. The method according to claim 1, wherein the colloidalsolution to be applied to the electrically conductive material isconcentrated or diluted with n-alkanols, n having a value between 1 and5, in order to alter the thickness of the coating.
 4. The methodaccording to claim 3, wherein the n-alkanol is ethanol.
 5. The methodaccording to claim 1, wherein the acid is acetic acid.
 6. The methodaccording to claim 1, wherein at least one of the alkali salts and theacid is diluted before addition to the colloidal solution in one or in amixture of a plurality of n-alkanols, n having a value between 1 and 5.7. The method according to claim 6, wherein the n-alkanol is ethanol. 8.The method according to claim 1, wherein the colloidal solution appliedto the electrically conductive material is tempered at temperatures upto 400° C.
 9. The method according to claim 1, wherein the colloidalsolution applied to the electrically conductive material is tempered attemperatures above 450° C. in air or oxygen.
 10. The method according toclaim 1, wherein the colloidal solution applied to the electricallyconductive material is tempered at temperatures above 650° C.
 11. Themethod according to claim 1, wherein properties of the alkali-containingglass are altered in a controlled manner by the addition of boric acidor of organic aluminum compounds.
 12. The method according to claim 1,wherein the properties of said alkali-containing glass are altered in acontrolled manner by the addition of chemical compounds selected fromthe group consisting of boric acid, boron compounds other than boricacid, and organic aluminum compounds.
 13. The method according to claim1, wherein the colloidal solution is applied to a first electricallyconductive material, a second electrically conductive material isdisposed on the first electrically conductive material such that thealkali-containing glass will be sandwiched between the first and thesecond electrically conductive material, and the electrical current isapplied across a boundary between the first electrically conductivematerial and the alkali-containing glass and a boundary between thesecond electrically conductive material and the alkali-containing glass.14. A colloidal solution for producing glass coatings suitable foranodic bonding to electrically conductive materials, the colloidalsolution being at least partly polymerized and comprising:a SiQ₂ -sol inone or in a mixture of n-alkanols, n having a value between 1 and 5; amixture of TEOS (tetraethyl orthosilicate) and MTEOS (methyltriethoxysilane); and an acid, water and alkali salts.
 15. The colloidalsolution according to claim 14, wherein the alkali salts are sodiumcompounds.
 16. The colloidal solution according to claim 14, wherein atleast one of the alkali salts and the acid is added dissolved in one orin a mixture of a plurality of n-alkanols, n having a value between 1and
 5. 17. The colloidal solution according to claim 16, wherein the oneor the mixture of a plurality of n-alkanols is selected from the groupconsisting of ethanol and methanol.
 18. The colloidal solution accordingto claim 14, wherein the acid is acetic acid.
 19. The colloidal solutionaccording to claim 14, wherein the colloidal solution contains boricacid or organic aluminum compounds.
 20. The colloidal solution accordingto claim 14, further including chemical compounds selected from thegroup consisting of boric acid, boron compounds other than boric acid,and organic aluminum compounds.
 21. The colloidal solution according toclaim 14, wherein the mass ratio of the mixture of TEOS and MTEOS isabout 1:3.